1. Field of the Invention
This invention relates to chrominance signal processing in a commercial video cassette recorder (VCR) and handles demodulation from a down converted chrominance signal into a color difference signal by digital signal processing.
2. Description of the Prior Art
Recently, various systems have been proposed to improve the performance of a VCR by introducing digital signal processing.
In the case of a chrominance signal, although a signal format is stipulated as a signal standard, there is no particular standard on signal processing. Therefore, a circuit configuration suitable for digital processing can be adopted with some flexibility.
A digital chrominance signal processing apparatus proposed for a VHS system is explained below.
For example, according to a Japanese patent publication no. 4-4799 (Feb. 12, 1985, "a chrominance signal processing apparatus") which is incorporated herein by reference, a reproduced down converted carrier chrominance signal is demodulated at a demodulator after being A/D-converted.
Phase error is detected from a color difference signal (R-Y) of the two demodulated color difference signals at a burst gate circuit. The error signal is D/A-converted, a clock signal corresponding to input jitter frequency is generated by controlling an analog variable frequency oscillator, and by applying the clock signal to the A/D converter and the demodulator mentioned above, a feedback APC (Automatic Phase Control) loop is completed.
But, in this feedback APC loop, if a quick response occurs, one signal processing becomes unstable and, moreover, when a cross-talk component exists in the reproduced signal such as in a long play mode, a comb filter is necessary in the loop. Thus, it is easy for the signal processing to become unstable.
Therefore, in the publication mentioned above, in order to improve upon the deficiency, an improvement of the response characteristic is attempted which includes adopting a feedforward APC in the output of the feedback APC. In particular, a phase error at a present line is calculated using burst signals and the color difference signals (R-Y) and (B-Y), and a vector operation to restore it to the normal demodulation axis is performed by an operation circuit.
A chrominance signal processing apparatus, as described in IEEE Trans. on CE, vol. 36, no. 3, Aug. 1990, p. 560-566, which is incorporated herein by reference, has basically the same configuration as the above-mentioned publication; however, it is different in that the feedback loop is digitally processed.
In the prior art, as a feedback APC is adopted for color demodulation, a high frequency jitter component cannot be removed. Moreover, when a comb filter exists in the loop, the response characteristic against jitter becomes worse.
Thus, if a color difference signal, in which the jitter component is not sufficiently removed, is passed through a comb filter, an intrinsic filter characteristic cannot be obtained and the ability to reject a cross-talk component and a noise component decreases.
As a result, because the detecting accuracy of a phase error from a burst signal decreases, the jitter suppressing effect becomes insufficient, even in the feedforward APC adopted in order to improve the response characteristic.